Joint fusion or arthrodesis is a common approach to alleviate the pain due to deteriorated and/or arthritic joints. Joint fusion involves inducing bone growth between two otherwise mobile bones in a joint, which alleviates pain by immobilizing and stabilizing the joint. The joint is generally fused in its most functional position. The ankle, wrist, finger, toe, knee and vertebral joints are all examples of joints that may be fused to alleviate pain associated with unstable, deteriorated joints.
The spinal motion segment consists of two adjacent vertebral bodies, the interposed intervertebral disc, as well as the attached ligaments, muscles, bony processes and the facet joints. The disc consists of the end plates at the surfaces of the vertebral bones, the soft inner core, called the nucleus pulposus and the annulus fibrosus ligament that circumferentially surrounds the nucleus and connects the vertebrae together. In normal discs, the nucleus cushions applied loads, thus protecting the other elements of the spinal motion segment. The nucleus in a normal disc responds to compression forces by bulging outward against the vertebral end plates and the annulus fibrosus. The annulus consists of collagen fibers and a smaller amount of elastic fibers, both of which are effective in resisting tension forces. However, the annulus on its own is not very effective in withstanding compression and shear forces.
As people age the intervertebral discs often degenerate naturally. Degeneration of the intervertebral discs may also occur in people as a result of degenerative disc disease. Degenerative disc disease of the spine is one of the most common conditions causing back pain and disability in our population. When a disc degenerates, the nucleus dehydrates. When a nucleus dehydrates, its ability to act as a cushion is reduced. Because the dehydrated nucleus is no longer able to bear loads, the loads are transferred to the annulus and to the facet joints. The annulus and facet joints are not capable of withstanding their increased share of the applied compression and torsional loads, and as such, they gradually deteriorate. As the annulus and facet joints deteriorate, many other effects ensue, including the narrowing of the interspace, bony spur formation, fragmentation of the annulus, fracture and deterioration of the cartilaginous end plates, and deterioration of the cartilage of the facet joints. The annulus and facet joints lose their structural stability and subtle but pathologic motions occur between the spinal bones.
As the annulus loses stability it tends to bulge outward and may develop a tear allowing nucleus material to extrude. Breakdown products of the disc, including macroscopic debris, microscopic particles, and noxious biochemical substances build up. The particles and debris may produce sciatica and the noxious biochemical substances can irritate sensitive nerve endings in and around the disc and produce low back pain. Affected individuals experience muscle spasms, reduced flexibility of the low back, and pain when ordinary movements of the trunk are attempted.
Degeneration of a disc is irreversible. In some cases, the body will eventually stiffen the joints of the motion segment, effectively re-stabilizing the discs. Even in the cases where re-stabilization occurs, the process can take many years and patients often continue to experience disabling pain. Extended painful episodes of longer than three months often leads patients to seek a surgical solution for their pain.
Several methods have been devised to attempt to stabilize the spinal motion segment. Some of these methods include: applying rigid or semi-rigid support members on the sides of the motion segment; removing and replacing the entire disc with an articulating artificial device; removing and replacing the nucleus; and spinal fusion involving permanently fusing the vertebrae adjacent the affected disc.
Spinal fusion is generally regarded as an effective surgical treatment to alleviate back pain due to degeneration of a disc. The fusion process requires that the vertebral endplates be prepared by scraping the surface of the existing vertebral bone to promote bleeding and release of bone growth factors, and placing additional bone or suitable bone substitute onto the prepared surface. Devices of an appropriate size made from rigid materials such as metals (including titanium and tantalum), some plastics (including polyetheretherketone (PEEK), or carbon fiber-filled PEEK), and allograft bone (primarily from donor femurs) are commonly inserted into the prepared disc cavity as part of the interbody fusion procedure to help distract and stabilize the disc space and put the vertebra into proper position while the bone growth process takes place. The interbody fusion procedure may be accomplished from an anterior, transforaminal, or a posterior surgical approach.
Most devices used in interbody spinal fusion require a relatively large opening that is typically larger than the dimensions of the rigid and unitary fusion device or cage that is to be inserted, examples of such devices include, U.S. Pat. No. 5,026,373 to Ray et al., U.S. Pat. No. 5,458,638 to Kuslich et al., and the NOVEL™ PEEK Spacers from Alphatec. In fact, many methods of interbody fusion, for example the method and device described in U.S. Pat. No. 5,192,327 to Brantigan, require bilateral placement of unitary devices through fairly large surgical openings. As with any surgical procedure, the larger the surgical access required, the higher the risk of infection and trauma to the surrounding anatomy.
There exists minimally invasive spinal fusion devices such as is disclosed in U.S. Pat. No. 5,549,679 to Kuslich and U.S. Pat. No. 6,997,929 to Manzi et al. The device disclosed in the U.S. Pat. No. 5,549,679 is a porous mesh bag that is filled in situ. The U.S. Pat. No. 6,997,929 is directed to a series of wafers that are vertically stacked to distract and support the vertebral endplates. U.S. Pat. No. 5,702,454 to Baumgartner discloses plastic beads which may be inserted one at a time into an intervertebral space on a flexible string. Further, U.S. Pat. No. 5,192,326 to Bao discloses hydrogel beads encased in a semi-permeable membrane.
While such minimally invasive technologies permit smaller access incision through the annulus (i.e. an annulotomy) to be used in a fusion procedure, the resulting fusion devices do not have the mechanical and dimensional features of the more rigid unitary fusion devices used in traditional surgical approaches and are less able to distract and stabilize the disc space. Thus, there is a need for a minimally invasive spinal fusion implant that could better emulate the mechanical and structural characteristics of a rigid unitary fusion device.